1. Technical Field
The present invention relates to a printing apparatus including a medium holding unit which is divided into a plurality of members to form a holding surface and which holds a printing medium being transported on the holding surface, a heat transfer type heating unit which is disposed in the medium holding unit and heats the printing medium held by the medium holding unit, and a printing head which performs printing by ejecting ink onto the printing medium.
In the specification, examples of the printing apparatus include an ink jet printer, a wire dot printer, a laser printer, a line printer, a copy apparatus, and a facsimile.
2. Related Art
In the past, a printing apparatus disclosed in Japanese Patent No. 3839316 included a printing head, a platen serving as a medium holding unit, and a heater serving as a heat transfer type heating unit.
Here, “the heat transfer type” refers to a method of transferring heat from a high temperature portion to a low temperature portion by passing it through the inside of an object. That is, the heat transfer type refers to a method of transferring heat by bringing the platen, which is the object into contact with a sheet, which is an example of the printing medium. The heat transfer type is also called “a contact type”.
The printing head is provided so as to perform printing by ejecting ink onto the sheet. The platen is provided so as to face the printing head and holds the sheet from the rear surface.
The heater is provided opposite the side of the platen holding the sheet and heats the sheet on the platen through the platen. The reason for heating the sheet on the platen is to promote the drying of the ink ejected onto the sheet. The platen is constituted by a plurality of members, since the platen expands due to the heat of the heater. In addition, narrow gaps are formed in the boundaries of the adjacent members. Therefore, since the gaps can absorb the expansion caused by the heat, it is possible to prevent the platen from being bent. As a consequence, the distance between the platen and the printing head can be maintained so as to be uniform.
FIGS. 11A and 11B are diagram illustrating a platen as an example of a known technique. FIG. 11A is the plan view illustrating the platen. FIG. 11B is the diagram illustrating the temperature of the positions of a sheet corresponding to the positions of the platen in a width direction of the platen shown in FIG. 11A.
The vertical axis represents the temperature. The horizontal axis represents the position of the sheet corresponding to the positions of the platen in the width direction of the platen.
As shown in FIG. 11A, a platen 200 according to a known technique includes a first member 201, a second member 202, and a third member 203. Gaps 205 are formed in the boundaries 204 of the adjacent members and screw holes 206 are screw-fixed to a plate-shaped member (not shown) provided on a lower side. A heater (not shown) for heating a sheet on the platen 200 is mounted so as to be interposed between the platen 200 and the plate-shaped member (not shown). The boundaries 204 are formed in a straight line so as to be inclined in a transport direction Y.
When the sheet passes through the platen 200 in the transport direction Y, as shown in FIGS. 11A and 11B, the sheet is heated by the heater and thus the temperature of the sheet increases.
A dot-dash line in FIG. 11B indicates the temperature of the sheet before the sheet passes through the platen 200. A solid line indicates the temperature of the sheet after the sheet passes through the platen 200.
Before the sheet passes through the platen 200, the temperature of the sheet is uniform in a width direction X.
When the sheet passes through the platen 200 in a transport direction Y, the heat of the heater can be transferred through the platen 200. Accordingly, the temperature of the sheet can be increased.
However, the temperature of the sheet passing through areas N where the boundaries 204 are formed in the width direction X is considerably lower than the temperature of the sheet passing through areas M where the boundaries 204 are not formed. Moreover, the irregularity in the temperature is large in the areas where the temperature of the sheet is low and the areas where the temperature of the sheet is high.
The areas M and N in the width direction X will be described.
The areas M are areas where there is no boundary 204 in the transport direction Y. On the other hand, the areas N are areas where there are the boundaries 204 in the transport direction Y.
Since the sheet passing through the areas M in the width direction X is not influenced by the gaps 205 of the boundaries 204, it is easy to be heated. Accordingly, the temperature of the sheet passing through the areas M is relatively high.
The sheet passing through the areas N passes through the boundaries 204 formed in the transport direction Y by a gap L of a component of the transport direction Y in each boundary 204 inclined in the transport direction Y. It is difficult for the sheet to be heated, since the sheet is not in contact with the platen 200 while the sheet passes through the boundaries. Accordingly, the temperature of the sheet passing through the areas N is lower than that of the sheet passing through the areas M. The larger the ratio of the gap L, which is the component of the transport direction Y to the length of the platen 200 in the transport direction Y, the larger the difference in the temperature.
The larger the inclination of the boundary 204 with respect to the transport direction Y, the lower the temperature over a board range. That is, the temperature becomes irregular over a broad range.
For this reason, an irregularity in the temperature and an irregularity in the dryness in the width direction X may be caused. Moreover, the irregularity in the temperature may cause discoloration of the ink.
The same applies to a case where the boundary is not inclined in the transport direction Y but is formed in a straight line parallel to the transport direction Y. In this case, the difference in the temperature is also significant.
When an end of the sheet is transported to and positioned on the boundaries in the case where the boundaries 204 are inclined in the transport direction Y, an end of the sheet may be stuck in the boundaries 204. Therefore, a problem arises in that the entire sheet passes obliquely or so-called sheet folding that the end of the sheet is curved occurs.